This application claims a priority based on Japanese Patent Application No. 2000-032343 filed on Feb. 9, 2000, the entire contents of which are incorporated herein by reference for all purposes.
The present invention relates to a mobile communication system and a radio base station apparatus, in which communication with a plurality of radio mobile station apparatuses is carried out using the code division multiple access method, and, in particular, relates to a mobile communication system and a radio base station apparatus, in which interference noise components contained in a target demodulation signal due to signals of other radio mobile station apparatuses are subject to suppression processing to obtain a signal that has been subjected to demodulation processing and that signal is transmitted toward a wired transmission path, thereby improving channel usage efficiency.
A wireless communication system using the code division multiple access method possesses suitable characteristics for mobile communication, such as resistiveness to mutual interference with another system, resistiveness to multi-path, easiness of hand-off, superiority in privacy protection, and the like. However, differently from other multiple access methods, in the communication system using the code division multiple access method, a plurality of users existing within a same base station cell carry out communication using a same frequency and a same frequency band at the same time, and accordingly, the system also has a problem of channel usage efficiency. This problem is caused as follows. Namely, interference is generated when, in the presence of the near-and-far problem, a strong signal of a user closer to a base station covers other weak communication signals. Or, interference by other stations is caused by unnecessary transmission power of a certain station itself. Such interference brings decrease of channel usage efficiency.
Countermeasures against decrease of channel usage efficiency include utilization of broad-band gain obtained by using broad-band signal transmission, use of a code series having low cross-correlation, adoption of power control for solving the near-and-far problem, and the like. In particular, power control not only suppresses unnecessary transmission power of a certain station itself leading its transmission power to the necessity minimum so as to suppressing interference by other stations to the minimum, but also is effective for solving the near-and-far problem.
However, even if these countermeasures are employed, it is difficult to prevent degradation of communication quality that accompanies increase of cross-correlation interference (degradation of signal-to-noise ratio) caused by increase of the number of users carrying out simultaneous communication.
As a measure for solving the above-mentioned problem, effective is an interference cancel technique that removes interference generated by other channels, and various types of interference cancel techniques have been examined. Interference by another user can be estimated in a receiver, and it is possible to perform processing for removing the interference. Among these interference cancel methods, there has been proposed a multi-user receiving method utilizing information on a plurality of user signals that are objects of demodulation. An interference cancel apparatus utilizing this method performs demodulation processing for a plurality of user signals that are objects of demodulation, performing respread processing and synthesizing processing on the demodulated signal to generate a replica signal, subtraction processing with the received multi-wave signal to perform demodulation processing, so as to suppress the interference components. Here, the interference cancel processing is performed in a cascaded multi-stage configuration, so that the interference component suppressing effect is increased.
FIG. 11 is a block diagram showing a configuration of an interference canceler unit according to the conventional technique. In the following, the conventional technique will be described referring to the drawing. In FIG. 11, the reference numeral 111 refers to an interference cancel processing part, 112 to a reverse spread processing/respread processing part, 113 to a synthesizing processing part, 114 to a subtraction processing part, and 115-1-115-n to demodulation processing parts.
As shown in FIG. 11, the interference canceler unit according to the conventional technique comprises: the interference cancel processing part 111 for performing interference cancel processing on a multi-wave signal into which a plurality of received spread spectrum signals are synthesized; and a plurality of demodulation processing parts 115-1-115-n for performing demodulation processing such as correlating detection and synchronous detection on the signal subjected to the interference cancellation. The interference cancel processing part 111 comprises: a plurality of reverse spread processing/respread processing parts 112 for performing reverse spread processing and respread processing on a received signal; synthesizing processing parts 113 for synthesizing signals subjected to the respread processing; and subtraction processing parts 114 for subtracting the synthesized signal from received signal. The interference cancel processing part 111 is constructed such that the processing by those processing parts is repeated in a plurality of stages. The interference cancel processing part 111 can process received signals of users #1-#n, and the demodulation processing parts 115-1-115-n perform the demodulation processing on the received signals of the users #1-#n, respectively.
Since the interference canceler unit according to the conventional technique has the above-described structure, the scale of the interference canceler unit and the quantity of processed signals become enormous. Further, in the case of a radio base station apparatus that should receive many users, the scale of the apparatus becomes unrealistically large when it is to be constructed to include an interference canceler unit that performs the interference cancel processing for all user signals.
As known information on this type of interference cancel, may be mentioned, for example, Japanese Non-examined Patent Laid-Open Nos. 10-51353, 10-190494, etc.
A mobile communication system using the above-described code division multiple access can not help increase of cross-correlation interference due to increase of the number of users in communication at the same time. Thus, interference cancel technique is indispensable. Among the interference cancel methods, the interference canceler method of multi-user receiving type utilizes information on a plurality of user signals that are objects of demodulation, and thus, requires a signal processing part including a plurality of correlators and respread circuits for suppressing cross-correlation interference components. This signal processing part for suppressing the cross-correlation interference components is constructed in multiple stages so as to improve the effect of suppressing the cross-correlation interference components, and accordingly, the scale of the apparatus becomes large. Further, the interference canceler system of the multi-user receiving type performs signal processing utilizing the user signals that are objects of demodulation. Accordingly, it is necessary to suitably manage and control each of the received signals. As a result, a communication system using thus-mentioned interference cancel method has such a problem that it becomes large in the scale of a base station apparatus and in the scale of signal processing, thus increasing costs.
An object of the present invention is to solve such problem in a mobile communication system using the above-mentioned code division multiple access method that the apparatus scale becomes larger as the number of received users becomes larger, in developing an interference cancel part for preventing decrease of channel efficiency owing to cross-correlation interference generated by increase of the number of communicating users. Further, another object of the present invention is to provide a mobile communication system and a radio base station apparatus that can effectuate high quality communication and prevent decrease in channel efficiency while moderating increase of the apparatus scale of the interference canceler part, and decreasing the apparatus scale of a radio base station apparatus and costs.
According to the present invention, the above-mentioned objects are attained by a mobile communication system provided with a radio base station apparatus communicating with radio mobile station apparatuses and a radio base station apparatus, wherein:
said radio base station apparatus comprises:
a signal processing means that receives a multi-wave signal as an input signal, into which spectrum spread codes from a plurality of radio mobile station apparatuses are synthesized, and that performs suppression processing for suppressing interference noise components other than a desired wave in demodulation of a target signal; and
a demodulation processing means for performing demodulation processing on spread signals that have been subjected to the suppression processing with respect to the interference noise components;
a number of user signals on which said signal processing means can perform signal processing is smaller than a number of user signals that have been subjected to said suppression processing for suppressing the noise components and that can be demodulated by said demodulation processing means; and
said signal processing means that suppresses the interference noise components selects a part of the user signals, and uses said selected part of the user signals to suppress the interference noise components.
In the above-described reception synchronous processing, as a method of selecting user signals that are to be subjected to reception processing in an interference canceler part of a receiving part of the radio base station apparatus, the methods described in the following may be used.
One method of selecting user signals that are to be subjected to the reception processing utilizes processing results of control channel communication processing that is performed at the time of registering a position of mobile station or at the time of starting to speak, decides allotment of user signals that are to be subjected to the interference cancel processing based on the result of the control channel communication processing, and delivers information required for starting the reception processing based on the result of allotment decision. In the code division multiple access radio communication system, transmission rates of signals used in communication are different between voice communication, data communication, etc. However, by multiplying spread signals of different spread ratios respectively, spread signals of a constant rate are generated. The spread signals are subjected to reverse spread processing on the receiving side. Since power control of signals is performed in a state of the spread signals, signal power including reverse spread gain after the reverse spread is different depending on the spread ratio. The fact that signal power differs depending on the spread ratio is noted and utilized to specify that a voice signal having a large reverse spread gain is not subjected to the interference cancel processing but to demodulation processing only, and a signal having a small reverse spread gain such as data communication is subjected to the interference cancel processing, for example.
Another method of selecting user signals that are to be subjected to the reception processing utilizes results of controlling an antenna apparatus that uses dynamically changeable directivity and gain of an antenna such as a smart antenna and array antenna to select target user signals that are to be subjected to the reception processing and user signals that are not targets, in order to select user signals that are to be subjected to the interference cancel processing. Namely, the antenna apparatus that utilizes directivity and gain of an antenna such as a smart antenna and array antenna to select user signals that are to be subjected to reception processing can select and distinguish individual user signals when communicating users are positioned in different directions from each other. On the other hand, when the user are positioned in the same direction although their distances are different, it is impossible to select and distinguish received user signals. Accordingly, the interference cancel processing is preferentially performed on those user signals, and information required for starting reception is delivered to the reception synchronous processing part.
Still another method of selecting user signals that are to be subjected to the reception processing utilizes results of path detection and switches selection circuits provided in the subsequent stage to the reception synchronous processing part, so as to perform signal allotting processing. Reception synchronous processing parts for respective received signals in the interference cancel part of the present invention perform synchronous processing and path detection processing on the received signals. In the path detection processing, delay profiles of radio transmission paths owing to convolution of the received signals are measured to detect a plurality of paths that is to be received by an RAKE synthesis part. The reception synchronous processing parts receive a reception start instruction once and deliver the results of the synchronous processing and the path detection to the control part. The control part switches selection circuits provided in the subsequent stage to the reception synchronous processing parts in order to perform the interference cancel processing preferentially with respect to mobile stations having large average peak power. This control is effective when a user signal has a difference in the average power level in comparison with other communication signals, even if power control is performed, such as in the case that the communicating user signal is extremely close or conversely distant.
Still another method of selecting user signals that are to be subjected to the reception processing utilizes the above-described methods in combination. Namely, the control part utilizes the results of control channel communication processing that is performed at the time of registering a position of mobile station or at the time of starting to speak and control information on dynamically changeable directivity and gain of an antenna such as a smart antenna and array antenna, in order to decide allotment of user signals as targets of the reception processing and non-target user signals. This allotment is delivered to the reception synchronous processing parts as information required for starting reception. Further, with respect to the user signals that have been once allotted, the control part utilizes results of the path detection in the reception synchronous processing parts that performed the synchronous processing and the path detection processing, in order to switch the selection circuits provided in the subsequent stage to the reception synchronous processing parts, thus performing signal switching processing.